1. Field of the Invention
This invention relates to a waste gas-treating catalyst for the removal of nitrogen oxides (NOx) from waste gas and more particularly to a waste gas-treating catalyst exhibiting notably improved frictional strength to the dust in the waste gas without a sacrifice of the removal activity of nitrogen oxides
2. Description of the Prior Art
Generally, the waste gases emanating from the boilers consuming heavy oil or coal as a fuel and the waste gases released from cement plants and sintering furnaces contain sulfur oxides (SOx). The catalysts which are used for the treatment of these waste gases, therefore, are required to be capable of withstanding the action of the SOx in addition to removing the NOx. From this point of view, it is generally held that titania type catalysts have a hopeful future.
In the case of the titania type catalysts, a catalyst of strength just fitting the application for which the catalyst is intended can be obtained by preparatorily molding a carrier material in a desired shape, sintering the molded material at an elevated temperature thereby forming a carrier, and depositing a catalytically active substance on the carrier as by coating or impregnation. The catalyst which is obtained by this method, however, suffers extreme smallness of both surface area and pore volume and encounters great difficulty in manifesting a high catalytic activity at low temperatures.
A highly active titania type catalyst enjoying largeness of both surface area and pore volume and manifesting high quality even at low temperatures at a high special velocity is preferable to be a molded catalyst. The molded catalyst, however, suffers smallness of strength and vulnerabibility of the gas-contact surface thereof to abrasion. When this catalyst is used for the purpose of treating a waste gas which contains dust copiously, the contact surface thereof is worn possibly to the extent of degrading the efficiency of treatment of waste gas and the powder consequently shed from the contact surface is discharged out of the system as entrained by the waste gas, with the possible result that the powder will induce secondary air pollution and clog the catalyst bed.
For the purpose of solving this problem encountered by the catalyst described above, a honeycomb type molded catalyst adapted to retain porosity throughout the entire volume thereof and endowed in the gas inlet side leading end thereof with improved wearproofness by means of sintering or vitreous coating has been proposed (U.S. Pat. No. 4,294,806).
This known catalyst indeed has an appreciably improved strength to withstand the wear caused by the dust in the waste gas. Since this molded catalyst has the inlet side leading end part thereof subjected to the impact of sintering or vitreous coating, however, it suffers loss of the catalytic activity, i.e. the removal activity of nitrogen oxides, in the affected part.
An object of this invention is, accordingly, to provide an improved catalyst to be used for the treatment of waste gas.
Another object of this invention is to provide a waste gas-treating catalyst having the wear-resisting strength thereof enhanced to a conspicuous extent without a sacrifice of the catalytic activity.